The pH of a food is also important in determining which organisms can survive and thrive on it. Many species of bacteria, including most pathogens, are inhibited by acidic conditions and cannot grow at a pH below 4.5. An exception is the lactic acid bacteria, which can grow at a pH as low as 3.5 and are used in the production of fermented foods such as yogurt and sauerkraut. This group of bacteria produces lactic acid as a result of fermentative metabolism. Although they are useful in food production, their growth and accompanying acid production is also a prime cause of spoilage of unpasteurized milk and other foods. Fungi can grow at a lower pH than most spoilage bacteria, leading to some foods, such as fruit, eventually becoming moldy. For example, the pH of lemons is approximately 2.2, which inhibits the growth of bacteria, including the lactic acid group, but some fungi can grow on them. ■ pH, p. 23 ■ lactic acid bacteria, p. 275 ■ pasteurization, p. 114

32.2 Factors Influencing the Growth of Microorganisms in Foods 803

The pH of a food product may also determine whether toxins can be produced. For example, Clostridium botulinum, the causative agent of botulism, does not grow or produce toxin below pH 4.5, and so it is not considered a danger in highly acidic foods. This is why the canning process for acidic fruits and pickles is less stringent than that for foods with a higher pH. However, some newer varieties of tomatoes are less acidic than older types, and require the addition of acid if they are to be safely canned using these less stringent procedures.

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